Iron and cobalt molecular complexes for the selective electrochemical reduction of co2 into co, with flow cells

ABSTRACT

The present invention concerns a flow cell electrolyzer (1) to electrochemically reduce reagent gas CO2 into gaseous CO, with an anodic compartment comprising an anode (2) with a current collector, and on the current collector, at least a catalyst to electrochemically oxidize H2O to O2, an anodic electrolyte solution (3) at a controlled flow rate Qa, comprising: a solvent, and an anodic electrolyte, the solvent being water at neutral or basic pH; a cathodic compartment comprising a cathodic electrolyte solution (6), at a controlled flow rate Qc, comprising: the solvent, and a cathodic electrolyte, the solvent being water at neutral or basic pH, a gas diffusion porous cathode (9) which comprises, on a gas diffusion porous current cathode collector which is electrochemically inert, at least a molecular catalyst on a surface S to electrochemically reduce CO2 into CO, with a by-production of H2, the molecular catalyst being chosen between the list with the metal chosen among: Iron, Cobalt: metal porphyrin with one or several +N(C1-C4 alkyl)3 groups, metal phthalocyanine, metal phthalocyanine with one or several +N(C1-C4 alkyl)3 groups, or cobalt quarter pyridine.

FIELD OF THE INVENTION

The present invention relates to iron and cobalt molecular complexes ascatalysts for the highly selective electrochemical reduction of CO₂ intoCO, with electrochemical cells comprising them.

STATE OF ART

Despite the increasingly frequent use of renewable energies to produceelectricity and avoid concomitant production of CO₂, it is reasonable toconsider that CO₂ emissions, in particular resulting from energyproduction, will remain high in the next decades. Thus, it appearsnecessary to find ways to capture CO₂ gas, either for storing orvalorization purposes.

Indeed, CO₂ can also be seen, not as a waste, but on the contrary as asource of carbon. For example, the promising production of syntheticfuels from CO₂ and water has been envisaged.

However, CO₂ exhibits low chemical reactivity: breaking its bondsrequires an energy of 724 kJ/mol. Moreover, CO₂ electrochemicalreduction to one electron occurs at a very negative potential, thusnecessitating a high energy input, and leads to the formation of ahighly energetic radical anion (CO₂ ^(●−)). Catalysis thus appearsmandatory in order to reduce CO₂ and drive the process tomulti-electronic and multi-proton reduction process, in order to obtainthermodynamically stable molecules. In addition, direct electrochemicalreduction of CO₂ at inert electrodes is poorly selective, yieldingformic acid in water, while it yields a mixture of oxalate, formate andcarbon monoxide in low-acidity solvents such as DMF.

CO₂ electrochemical reduction thus requires catalytic activation inorder to reduce the energy cost of processing, and increase theselectivity of the species formed in the reaction process.

Molecular catalysts that combine high selectivity and high currentdensity for CO₂ electrochemical reduction to CO or other chemicalfeedstocks are in high demand. Molecular transition metal catalystsoffer the distinct advantage of allowing for the fine-tuning of theprimary and secondary coordination spheres by manipulating the chelatingenvironment and the steric and electronic effects of the ligands. Theability to improve catalytic efficiency and product selectivity throughthe rational optimization of the ligand structure is a feature notaccessible to the solid-state catalysts common to pilot-scaleelectrolyzer units.¹⁻² There is now a range of known molecularcatalysts, including those based on noble (e.g. Ru, Ir and Re) andearth-abundant metals (e.g. Co, Ni, Fe, Mn and Cu).¹⁻⁹ These catalyststypically trigger a two electron reduction of CO₂ to either CO orformate with reasonable efficiencies, but in organic solvents.

The integration of above described molecules by applying thin porouscarbon films, such as carbon powder, carbon nanotubes or graphene toform hybrid catalytic materials, has proven to be a promising strategyto selectively achieve CO production in pure aqueous conditions.Reasonable performances have been obtained in nearly neutral conditions(pH 7-7.5) with good selectivity.^(10,11) While these performancesrepresent advances for CO₂RR (CO₂ reduction reaction) catalysts, muchhigher current densities are required for commercial operation, beinghighly selective and operating at low overpotentials at the same time.Moreover, these current densities remain far below those obtained withstate-of-the-art solid-state Ag^(12,13) or Au¹⁴ nanomaterials have beenreported to reach >150 mA/cm².

SUMMARY OF THE INVENTION

Here, the invention presents a flow cell electrolyzer toelectrochemically reduce a gas reactant comprising CO₂, into gaseous COand gaseous H₂, with:

an anodic compartment comprising:

-   -   an anode with a current collector, and optionally on the current        collector, at least a catalyst to electrochemically oxidize H₂O        to O₂,    -   an anodic electrolyte solution, at a controlled flow rate Q_(a),        comprising: a solvent, and an anodic electrolyte, the solvent        being water,    -   an anodic electrolyte solution inlet and an anodic electrolyte        solution outlet connected to the anodic compartment, to        circulate the anodic electrolyte solution;

a cathodic compartment comprising:

-   -   a cathodic electrolyte solution, at a controlled flow rate        Q_(c), comprising: a solvent, and a cathodic electrolyte, the        solvent being water,    -   a gas diffusion porous cathode which comprises, on a gas        diffusion porous current cathode collector which is        electrochemically inert, at least a molecular catalyst        incorporated in the porous cathode with a surface S, to        electrochemically reduce the gas comprising CO₂, into gaseous CO        with a by-production of gaseous H₂,        the molecular catalyst being chosen between the list:    -   metal porphyrin with one or several ⁺N(C₁-C₄ alkyl)₃ groups,        with the metal chosen among: Iron, Cobalt;    -   metal phthalocyanine, with the metal chosen among: Iron, Cobalt;    -   metal phthalocyanine with the metal chosen among: Iron, Cobalt,        with one or several groups among: ³⁰ N(C₁-C₄ alkyl)₃, F, C(CH₃),        or    -   cobalt quarter pyridine;

a channel for flowing the reagent gas CO₂, at a controlled flow rate Qg,onto or through the surface S of the gas diffusion porous currentcathode collector;

a cathodic electrolyte solution inlet and a cathodic electrolytesolution outlet (8) connected to the cathodic compartment, to circulatethe cathodic electrolyte solution, and the remaining reagent gas CO₂ andthe product gas CO by the outlet;

an anion exchange membrane, impermeable to CO₂, CO, H₂ and O₂, betweenthe anodic compartment and the cathodic compartment;

Pumping means serving to:

-   -   circulate by pumping the anodic electrolyte solution and the        cathodic electrolyte solution between the inlet and the outlet,    -   flow by pumping the comprising gas CO₂ in the channel through        the gas diffusion porous cathode; the pumping means being        configured to control all flow rates of the cathodic and anodic        electrolytes as well as the reagent gas CO₂ passing through the        surface S of the gas diffusion porous cathode;

a power supply providing the energy necessary to trigger theelectrochemical reactions involving the reagent.

At the anode, several oxidation reactions can occur, for instance:

-   -   the Oxygen Evolution Reaction, producing O₂ from H₂O, or    -   alcohol oxidation reactions.

The invention presents also a method of reducing a gas reactantcomprising CO₂ into gazeous CO, with the flow cell electrolyzercomprising:

-   -   passing (diffusing or projecting) the flow of reagent gas CO₂,        at a controlled flow rate Q_(g), onto or through the surface S        of the gas diffusion porous current diffusion electrode (GDE) of        the cathodic compartment on which there is the molecular        catalyst,    -   while the catholyte electrolyte solution circulates in between        the Gas Diffusion Electrode and the anion exchange membrane, and

applying a potential to the Gas Diffusion Electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the disclosed devices andmethods will become apparent from reading the description, illustratedby the following figures, where:

FIG. 1. Current density and CO selectivity recorded at variouspotentials. Recorded in CO₂ saturated 0.5 M NaHCO₃ (pH 7.3). Eachpotential step was maintained for 20 min.

FIG. 2. Current density and CO selectivity recorded during a 24-hourelectrolysis at −0.78 V vs RHE in CO₂ saturated 0.5 M NaHCO₃. The cellpotential was 3.41 V.

FIG. 3. Applied potential and CO selectivity recorded underchronopotentiometric conditions at 50 mA·cm⁻² for 3 hours in CO₂saturated 0.5 M NaHCO₃. The cell potential was 4.05 V.

FIG. 4. Applied potential and CO selectivity recorded underchronopotentiometric conditions at 50 mA·cm⁻² for 3 hours in CO₂saturated 1.0 M NaHCO₃. The cell potential was 3.46 V.

FIG. 5. (A) Current density and CO selectivity at various celltemperatures. Recorded in CO₂ saturated 0.5 M NaHCO₃. (B) Arrhenius plotconstructed from the TOF values calculated for each temperature step in(A).

FIG. 6. Current density and CO selectivity recorded at variouspotentials. Recorded in 1.0 M KOH solution (pH 14). Each potential stepwas maintained for 20 min.

FIG. 7. Applied potential and CO selectivity recorded underchronopotentiometric conditions at 27 mA·cm⁻² for 24 hours in 1.0 M KOH.The cell potential was 1.87 V.

FIG. 8. Applied potential and CO selectivity recorded underchronopotentiometric conditions at 50 mA·cm⁻² for 3 hours in 1.0 M KOH.The cell potential was 2.36 V.

FIG. 9. Step potential experiment, giving the current density recordedat various potentials. Recorded in CO₂ saturated 0.5 M NaHCO₃ (pH 7.3).Each potential step was maintained for 20 min.

FIG. 10. Step potential experiment, giving the current density recordedat various potentials. Recorded in argon saturated 0.5 M NaHCO₃ (pH 7.3)at a film formulated with the non-metalated porphyrin ligand. Eachpotential step was maintained for 20 min.

FIG. 11. Current density and CO selectivity recorded during a 6 hourelectrolysis at −0.78 V vs RHE in CO₂ saturated 1 M NaHCO₃.

FIG. 12. Step potential experiment, giving the current density recordedat various potentials. Recorded in 1 M KOH (pH 14). Each potential stepwas maintained for 20 min.

FIG. 13. Step potential experiment, giving the current density recordedat various potentials. Recorded in 1 M KOH (pH 7.3) at a film formulatedwith the non-metalated porphyrin ligand. Each potential step wasmaintained for 20 min.

FIG. 14a . Cross-sectional view (a) and general scheme (b) of the CO₂electrolyzer flow cell.

FIG. 14b . Scheme on the operation of the CO₂ electrolyzer flow cell ofthe present invention, which represents the stream (jet) of CO₂projected on a Gas Diffusion Electrode.

FIG. 15. a: Current density for CO production as a function of thepotential. b: Bulk electrolysis at fixed potential (E=−0.72 V vs. RHE)for CoPc2@carbon black deposited onto a carbon paper as cathodicmaterial, in 1 M KOH. c: Co K-edge XANES profiles of CoPc2 (black dots),and CoPc2@carbon black before and after electrolysis (E=−0.72 V vs. RHE)which are superposed, in 1 M KOH solution.

FIG. 16. Flow cell electrolyzer set-up.

FIG. 17. Repetitive cyclic voltammetry for CoPc1@MWCNTs (bottom) andCoPc2@MWCNTs (top) films deposited onto a glassy carbon electrode (d=3mm) in 0.5 M NaHCO₃ solution saturated with CO₂ (pH 7.3; black, 1^(st)scan; grey, 2^(nd) scan), at v=0.1 V s⁻¹.

FIG. 18. SEM image of a catalytic film deposited onto carbon paper.

FIG. 19. Blank experiments: Left, at E=−0.676 V vs. RHE) in the absenceof catalyst in CO₂ saturated 0.5 M NaHCO₃ solution (pH=7.3); right, atE=−0.605 V vs. RHE with CoPc2@MWCNTs (1:15) in Ar saturated 0.5 M NaHCO₃solution (pH=8.5). In both cases, only H₂ was detected as reactionproduct.

FIG. 20. Variation of the total current density as a function of theelectrolysis potential at optimized mass ratio for CoPc1@MWCNTs in a CO₂saturated solution containing 0.5 M NaHCO₃ (pH 7.3).

FIG. 21. Co 2p (left) and N 1s (right) XPS spectra before and after a 7h electrolysis (E=−0.676 V vs. RHE) with CoPc2@MWCNTs with a 1:15 ratioin a CO₂ saturated solution with 0.5 M NaHCO₃ (pH 7.3).

FIG. 22. Bulk electrolysis at E=−0.971 V vs. RHE of a CO₂ saturatedsolution containing 0.5 M KCl (pH 4) using CoPc2@MWCNTs as catalyst.

FIG. 23. Total current density as a function of the applied potentialfor CoPc2@carbon black deposited onto a carbon paper as cathodiccatalytic material, in 1 M KOH solution (flow cell device, see maintext). Each potential step was held for a duration of 20 min.

FIG. 24. Top: K-space (insert) and Fourier transform EXAFS spectra ofCoPc2@carbon black before (black) and after electrolysis (E=−0.72 V vsRHE) (grey) in 1 M KOH solution. Bottom: Co K-edge XANES spectra of theCoPc2@carbon black electrode after electrolysis (E=−0.72 V vs RHE) (red)together with those of reference compounds CoO (purple), CoOOH (orange),Co₃O₄ (blue) and metallic Co (green).

FIG. 25. Bulk electrolysis at fixed current density (75 mA cm⁻²) withCoPc2@carbon black deposited onto a carbon paper as cathodic catalyticmaterial, in 1 M KOH solution (flow cell device, see main text). Duringthe electrolysis, 2.5 g KOH were added every half hour into the cathodesolution to maintain the pH balance.

FIG. 26. CO Selectivity as a function of time in hours at 50 mA/cm² withCo(qpy) catalyst in a zero gap cell device.

DETAILED DESCRIPTION

Inventors have developed a new flow cell electrolyzer that surprisinglyallows an efficient reduction of CO₂ into gazeous CO with particularlyhigh current densities and selectivity while operating at lowoverpotentials.

Accordingly, the present invention concerns: a flow cell electrolyzer 1to electrochemically reduce a gas reactant (in a form of a streamprojected on a GDE) comprising CO₂, into gaseous CO, with:

an anodic compartment comprising:

-   -   an anode 2 with a current collector, and, optionally, on the        current collector, at least a catalyst to electrochemically        oxidize H₂O to O₂,    -   an anodic electrolyte solution 3, at a controlled flow rate        Q_(a), comprising: a solvent, and an anodic electrolyte, the        solvent being water, the anodic compartment being connected to        an anodic electrolyte solution inlet 4 and an anodic electrolyte        solution outlet 5;

a cathodic compartment comprising:

-   -   a cathodic electrolyte solution 6, at a controlled flow rate        Q_(c), comprising: a solvent, and a cathodic electrolyte, the        solvent being water, the cathodic compartment being connected to        a cathodic electrolyte solution inlet 7 and a cathodic        electrolyte solution outlet 8,    -   a gas diffusion electrode 9 (porous cathode) comprising on an        electrochemically inert gas diffusion porous current collector        of surface S, at least a molecular catalyst to electrochemically        reduce the gas flow comprising CO₂ into gaseous CO, with        by-production of gaseous H₂ and O₂,

The cathodic electrolyte can comprise a phosphate buffer or potassiumhydroxide, and the anodic electrolyte comprises a phosphate buffer orpotassium hydroxide.

Also, the electrolyte can be sodium hydroxide NaOH (Nat) or cesiumhydroxide CsOH (Cs⁺).

The cathodic electrolyte can comprise sodium hydroxide NaOH or cesiumhydroxide CsOH and/or the anodic electrolyte comprises sodium hydroxideNaOH or cesium hydroxide CsOH.

The molecular catalyst is chosen in the list:

-   -   metal tetra phenyl porphyrin with at least one or several        ⁺N(C₁-C₄ alkyl)₃ groups, with the metal chosen among: Iron,        Cobalt; the other groups are independently selected from the        group consisting of H, OH, F, C(CH₃);    -   metal phthalocyanine, with the metal chosen among: Iron, Cobalt;    -   metal phthalocyanine with the metal chosen among: Iron, Cobalt,        with one or several groups among: ⁺N(C₁-C₄ alkyl)₃, F, C(CH₃)₃,        the other groups are H;    -   cobalt quarter pyridine.

The flow cell electrolyzer 1 comprises also:

an anion exchange membrane 10, impermeable to CO₂, CO, H₂ and O₂,between the anodic compartment and the cathodic compartment;

a channel 11 for flowing the reagent gas CO₂, at a controlled flow rateQ_(g), through the surface S of the gas diffusion porous current cathodecollector;

a power supply providing the energy necessary to trigger theelectrochemical reactions involving the reagent.

On the current collector of the anode, different reactions can occur.For instance:

at least one catalyst electrochemically oxidizes H₂O to O₂; or

at least one catalyst electrochemically oxidizes alcohols to organiccompounds such as esters.

In the flow cell electrolyzer 1, all flow rates are controlled bypassing the cathodic and anodic electrolytes as well as the reagent gasthrough pump means 12 serving to:

-   -   circulate by pumping the anodic electrolyte solution 3 and the        cathodic electrolyte solution 6 between the inlets 4, and the        outlets, 5,    -   flow by pumping the flow of reagent gas CO₂ onto or through the        gas diffusion porous cathode 9;

In an example, the gas reactant comprises at least 99% per volume CO₂,or at least 99.5% per volume CO₂, or at least 99.9% per volume CO₂, orat least 99.99% per volume CO₂.

The flow cell 1 can have a device capable of recording the currentgenerated by the electrochemical processes.

Advantageously, but in a non-limiting way, the channel for passing(diffuses or projects through or on the porous surface S of the GDE, thestream of CO₂) the reagent gas CO₂ can comprise a flow frame generatinga turbulent flow.

Advantageously, but in a non-limiting way, pumping means 12 torecirculate the anodic electrolyte solution 3 and the cathodicelectrolyte solution 6.

In a preferred embodiment, the anodic and/or cathodic electrolytesolution has a neutral pH (for instance a pH comprised between 6.5 and7.5), or a basic pH.

In yet another preferred embodiment, the cathodic electrolyte solutionhas a basic pH. In such an embodiment, the anodic electrolyte solutionmay have an acidic or neutral pH.

In particular, the anodic electrolyte solution and/or the cathodicelectrolyte solution has a pH from 9 to 14, preferably from 10 to 14,more preferably from 11.5 to 14, and more preferably from 13 to 14.

In yet another preferred embodiment, the cathodic electrolyte solutionhas a pH from 9 to 14, preferably from 10 to 14, more preferably from11.5 to 14, and more preferably from 13 to 14. In such an embodiment,the anodic electrolyte solution may have an acidic or neutral pH.

Basic, i.e. alkaline conditions, in particular when applied to thecathodic electrolyte solution, allow operating the flow cellelectrolyzer at surprisingly high current densities in comparison toacidic conditions.

The combination of basic conditions with the aforementioned molecularporphyrin, phthalocyanine or quarter pyridine catalysts further resultin surprisingly high selectivities for the desired conversion of CO₂ toCO, with only minor or even no side product formation. Thus, with thehigh current densities and high CO selectivity, the invention allowsproduction of close-to-pure CO from CO₂ at high throughput rates. Theelectrochemical reaction occurs at the triple phase interface betweenthe catalyst (solid), the gaseous CO₂ (gas), and the catholyte (liquid).The use of gaseous CO₂ streaming through a porous cathode enables highcurrent densities.

Furthermore, it was found that the aforementioned molecular catalystsdemonstrate high long-term stability even at harsh alkaline pHconditions.

Further, the flow cell electrolyzer of the invention is effective undersmooth conditions of temperature, pressure, and uses aqueouselectrolytes. Such operating conditions are particularly advantageous interms of, e.g., ease of use and energy consumption.

The electrochemical reduction of the reagent into CO can be carried outat ambient temperature. Also, in the flow cell electrolyzer 1 of theinvention, the anodic and the cathodic electrolytes are at ambienttemperature, In another realization mode, the anodic and the cathodicelectrolytes are at ambient temperature are at a temperature higher thanambient temperature.

The electrochemical reduction of the reagent into CO can be carried outat atmospheric pressure. Also, in the flow cell electrolyzer 1 of theinvention, the gas reactant flow is at atmospheric pressure. In anotherrealization mode, the anodic and the cathodic electrolytes are at apressure higher than atmospheric pressure.

Advantageously, the cathodic current collector can be carbon paper,stainless steel or other material. Advantageously, if it is on carbonpaper, a composite multilayered porous current collector (for instance apolytetrafluoroethylene gas distribution layer) can be filed on thecarbon paper to increase the reduction rate/efficiency.

In a realization, the porous cathode 9 comprises on the currentcollector an electrode film which, at least, contains polymers and themolecular catalysts. The electrode film can be filed in or grafted onthe current collector.

Advantageously, when the molecular catalyst is a metal porphyrin, themetal porphyrin is a tetraphenylporphyrin which comprises specificgroups on the phenyl moieties, with at least one or several ₊N(C₁-C₄alkyl)₃ groups among specific groups.

In a first embodiment of the flow cell electrolyzer 1, the molecularcatalyst is the iron porphyrin with the formula:

Wherein:

-   -   at least 1 and at most 8 groups among R₁ to R₁₀ and R_(1′) to        R_(10′) being independently ⁺N(C₁-C₄ alkyl)₃ group,    -   the remaining groups R₁ to R₁₀ and R_(1′) to R_(10′) are        independently selected from the group consisting of H, OH, F,        C(CH₃)₃

In particular, the iron molecular catalyst can comprise:

the other groups among R₁ to R₁₀ and R_(1′) to R_(10′) are H; or

-   -   the other groups among R₁ to R₁₀ and R_(1′) to R_(10′) are H and        F;

The iron molecular catalyst can comprise the ⁺N(C₁-C₄ alkyl)₃ groups inpara or ortho position.

For instance: the molecular catalyst is an iron porphyrin FeTNT offormula (II):

Or chosen among these iron containing molecules:

preferably as its chloride salt.

With the iron porphyrin as molecular catalyst, at the operationalconditions: the pH is between 7.3 and 14, and the potential applied tothe cathode 9 is between 0 V and −1 V versus RHE (Reversible HydrogenElectrode), the results of the flow cell can be: a current density forCO production of more than 5 and at least 150 mA·cm⁻², and theselectivity of the electrochemical reaction which is between 98% and99.9%.

Also in a more particular embodiment, in the flow cell electrolyzer 1according to the invention, with a tetra-phenyl iron porphyrin as amolecular catalyst:

the pH is between from 11.5 to 14, more preferably from 13 to 14,

the gas reactant flow passes at atmospheric pressure through the poroussurface S of the gas diffusion electrode (GDE), and

the anodic and the cathodic electrolytes are at ambient temperature.

In a second embodiment of the flow cell electrolyzer 1, the flow cellelectrolyzer 1 comprises the molecular catalyst which presents theformula:

wherein R₁ to R₁₆ are independently selected from the groups consistingof H, F, C(CH₃)₃ or ⁺N(C₁-C₄ alkyl)₃

In particular, R₁ to R₁₆ are H, and the molecular catalyst is a cobaltphthalocyanine CoPc of formula:

With the phthalocyanine as molecular catalyst, at the operationalconditions: the pH is between 7.3 and 14, the potential applied to thecathode 9 is between −0.48 V and −0.98 V versus RHE, the results of theflow cell can be: a current density for CO production of more than 10and at least 50 mAcm⁻², and a selectivity of the electrochemicalreaction between 90% and 92%.

In another variant, the cobalt molecular catalyst presents at least 1and at most 8 groups among R₁ to R₁₆ being independently ⁺N(C₁-C₄alkyl)₃ group.

In another variant, the cobalt molecular catalyst presents for one orseveral of the specific following R₁, R₄, R₅, R₈, R₉, R₁₂, R₁₃ and R₁₆groups, a ⁺N(C₁-C₄ alkyl)₃ substituent.

For instance, the molecular catalyst can be a cobalt phthalocyanineCoPc2 of formula:

or the molecular catalyst is a cobalt phthalocyanine CoPc3 of formula:

With the cobalt phthalocyanine with one or several ⁺N(C₁-C₄ alkyl)₃groups, as molecular catalyst, at the operational conditions: the pH isbetween 7.3 and 14, and the potential applied to the cathode 9 isbetween −0.3 V and −1 V versus RHE, the results of the flow cell can be:a current density for CO production of more than 10 and at least 150mAcm⁻², and a selectivity of the electrochemical reaction is between 92%and 96%.

Also, in a more particular embodiment, in the flow cell electrolyzer 1according to the invention, with a cobalt phthalocyanine as a molecularcatalyst, wherein:

the pH is between from 11.5 to 14, more preferably from 13 to 14,

the gas reactant flow passes at atmospheric pressure through the poroussurface S of the gas diffusion electrode (GDE), and

the anodic and the cathodic electrolytes are at ambient temperature.

In an even more particular embodiment, in the flow cell electrolyzer 1according to the invention, with a cobalt phthalocyanine with one orseveral ⁺N(C₁-C₄ alkyl)₃ groups, wherein:

the pH is between from 11.5 to 14, more preferably from 13 to 14,

the gas reactant flow passes at atmospheric pressure through the poroussurface S of the gas diffusion electrode (GDE), and

the anodic and the cathodic electrolytes are at ambient temperature.

In a third embodiment of the flow cell electrolyzer 1, the flow cellelectrolyzer 1 comprises the molecular catalyst which presents theformulae of the cobalt quarter pyridine

With the cobalt quarter pyridine as molecular catalyst, at theoperational conditions: the pH is between 7.3 and 14, and the potentialapplied to the cathode 9 is between −0.6 V and −1 V versus RHE, theresults of the flow cell can be: a current density for CO production isbetween 25 and 200 mAcm⁻², and the selectivity of the electrochemicalreaction is between 86.1% and 98.8%.

In a particular embodiment, in the flow cell electrolyzer 1 according tothe invention, with a cobalt quarter pyridine as molecular catalyst,wherein:

the pH is between from 11.5 to 14, more preferably from 13 to 14,

the gas reactant flow passes at atmospheric pressure through the poroussurface S of the gas diffusion electrode (GDE), and

the anodic and the cathodic electrolytes are at ambient temperature.

As shown in the experimental section, flow cell electrolyzer of theinvention allows an efficient reduction of CO₂ into gazeous CO withparticularly high current densities and selectivity while operating atlow overpotentials. Without wishing to be bound by any theory, thiscould stem from the fact that the reaction occurs at the triple phaseinterface between the molecular catalyst supported on the electrode(solid), the reagent CO₂ (gas) and the electrolyte solution (liquid).

Invention also relates to a method of reducing a gas reactant comprisingCO₂ into gazeous CO, said method comprising, in a flow cell electrolyzer1 as defined above:

-   -   passing the flow of reagent gas CO₂, at a controlled flow rate        Q_(g), onto or through the surface S of the gas diffusion porous        current diffusion electrode (GDE) of the cathodic compartment on        which there is the molecular catalyst,        while the cathodic electrolyte solution 6 circulates in between        the Gas Diffusion Electrode 9 and the anion exchange membrane        10, and    -   applying a potential at the Gas Diffusion Electrode 9.

In an embodiment, in said method, the anodic and/or cathodic electrolytesolution has a neutral (e.g. comprised between 6.5 and 7.5) or a basicpH. In a particular embodiment, the anodic electrolyte solution and/orthe cathodic electrolyte solution has a pH comprised from between 9 to14, preferably from 10 to 14, more preferably from 11.5 to 14, and evenmore preferably from 13 to 14.

In another embodiment, in said method, the gaz reactant flow is atatmospheric pressure. In another embodiment the gas reactant flow is ata pressure higher than atmospheric pressure.

In an embodiment, in the method according to the invention, the anodicand the cathodic electrolytes are at ambient temperature. In anotherembodiment, the anodic and the cathodic electrolytes are at atemperature higher than ambient temperatures.

The flow cell electrolyzer of the invention allows to combine highselectivity and high current density for CO₂ electrochemical reductionto CO. Accordingly, in an embodiment, in the method of the invention thepotential applied to the Gas Diffusion Electrode 9 of the flow cellelectrolyzer of the invention is of:

between 0 V and −1 V versus RHE, when the molecular catalyst is atetra-phenyl iron porphyrin, thereby generating a current density for COproduction of at least 150 mA·cm⁻²;

between 0.48 V and −0.98 V versus RHE when the molecular catalyst is ametal phthalocyanine, thereby generating a current density for COproduction of at least 50 mA·cm⁻²;

between −0.3 V and −1 V versus RHE when the molecular catalyst is acobalt phthalocyanine with one or several ⁺N(C₁-C₄ alkyl)₃ groups,thereby generating a current density for CO production of at least 150mA·cm-2;

between −0.6 V and −1 V versus RHE when the molecular catalyst is acobalt quarter pyridine with one or several ⁺N(C₁-C₄ alkyl)₃ groups,thereby generating a current density for CO production of at least 150mA·cm⁻²

In another particular embodiment, when the potential applied to the GasDiffusion Electrode 9 of the flow cell electrolyzer of the invention isof between 0 V and −1 V versus RHE, when the molecular catalyst is atetra-phenyl iron porphyrin, said potential is not 0 V, therebygenerating a current density for CO production of at least 150 mA·cm⁻².

In a more particular embodiment, in said method:

when the molecular catalyst is a tetra-phenyl iron porphyrin, theselectivity of the electrochemical reaction is between 98% and 99.9%;

when the molecular catalyst a metal phthalocyanine, the selectivity ofthe electrochemical reaction is between 90% and 92%;

when the molecular catalyst is a cobalt phthalocyanine with one orseveral ⁺N(C₁-C₄ alkyl)₃ groups,

the selectivity of the electrochemical reaction is between 92% and 96%;

when the molecular catalyst is a cobalt quarter pyridine with one orseveral ⁺N(C₁-C₄ alkyl)₃ the selectivity of the electrochemical reactionis between 86.1% and 98.8%.

First Embodiment

Preparation of the Hybrid Materials for the Gas Diffusion Electrode withFeTNT

7.2 mg carbon black was dispersed in 8 mL ethanol by sonication for 30min. A solution of 2.08 mg FeTNT in 2 mL ethanol was added to the carbonblack suspension followed by sonication for 30 min. 20 μL of a 5% w/wNafion solution was added followed by sonication for 30 min. The asprepared ink was disposed at 65° C. on carbon paper masked with a PTFEframe to obtain a 1×1 cm² gas diffusion current collector.

Results

The inventors included FeTNT into a flow cell setup comprising FeTNTsupported on a gas diffusion electrode as the cathode 9. Details of thesetup are provided in the Supporting Information (see also FIGS. 14 and16). Briefly, the electrolyzer consists of a sandwich of flow frames,electrodes, gaskets and an ion exchange membrane 10, which wereassembled as schematically illustrated in FIG. 14a . A gas flow of CO₂is delivered from the back side of the cathodic compartment and flowsthrough the gas diffusion electrode (GDE), while the catholyte solutionis circulated in between the GDE and the anion exchange membrane 10(AEM). On the other side of the AEM, the anolyte is directed between theAEM and the Pt/Ti alloy anode 2, FIG. 14b . FeTNT was dispersed in acolloidal ink with carbon black and subsequently deposited on the carbonfiber paper, composing the cathode 9.

A survey of the catalytic activity as a function of applied potentialwas first performed in CO₂ saturated 0.5 M NaHCO₃. FIG. 1 illustratesthe increase in current density recorded for the CO₂RR as theoverpotential was gradually set to more negative values by steps of 100mV, along with the selectivity for CO production (see also FIG. 9).Initially, at the less negative potentials, CO was detected as the onlyproduct; whereas minor amounts of H₂ stemming from the HER (HydrogenEvolution Reaction) was observed at more negative potentials. The smallamounts of the latter, even at potentials well beyond the HER onset,reflects the ability of the catalyst to strongly suppress the HER infavour of CO production. This was further verified by employing anon-catalytic film in the electrolyzer, i.e., a film formulated with thenon-metalated porphyrin ligand, resulting in H₂ production exclusively(see FIG. 10). The average current density and the CO selectivity ateach applied potential is collected in Table 1 hereafter.

TABLE 1 Current densities and CO selectivity obtained with a catalyticfilm with FeTNT as catalyst (pH 7.3) as a function of the potentialapplied at the cathode 9. E/V vs RHE j_(av)/mAcm⁻² % CO −0.38  7.2 99.9−0.48  9.8 99.9 −0.58 14.8 99.8 −0.68 20.7 99.5 −0.78 27.2 99.3 −0.8834.9 98.9 −0.98 42.1 98.6

The endurance of the catalytic film was tested by performing achronoamperometric electrolysis for 24 hours. Based on the values listedin Table 1, a potential of −0.78 V vs RHE was chosen as this potentialcompromises between high current density and the ability of the catalystto provide a nearly perfect CO production. The current density and theCO selectivity that have been obtained are shown in FIG. 2,demonstrating the excellent stability of the system: a current densityof 27.3 mAcm⁻² was maintained through the experiment with an averageselectivity for CO of 98.2±0.2%.

Under the same conditions, the cell performance was further evaluated byperforming a chronopotentiometric electrolysis at a current density of50 mAcm⁻². As shown in FIG. 3, the cell maintained this current valuefor 3 hours at a potential of −1.14 V vs RHE. The product selectivitywas not affected by the almost 2-fold current density increase, as COwas produced with a 98.3±0.3% average selectivity.

In order to diminish the cell potential, the ohmic resistance of thecell was reduced by increasing the electrolyte concentration. This wasdemonstrated in a 1.0 M NaHCO₃ electrolyte solution, where a gain of >8mAcm⁻² was obtained at an applied potential of −0.78 V vs RHE, withoutcompromising the CO selectivity (see FIG. 11). In a chronopotentiometricelectrolysis, see FIG. 4, the applied potential at 50 mAcm⁻² shiftedpositively to −0.86 V vs RHE, and in addition the cell potential wasreduced by 590 mV. The CO product selectivity was 99.0±0.2% on average.

A complementary approach to boost the cell performance was achieved bythermally activating the catalyst. During a chronoamperometricelectrolysis at −0.78 V vs RHE, the catholyte temperature wasincrementally increased from 24° C. to 34° C. and finally to 40° C. Asshown in FIG. 5A, an increase in current density was observed uponincreasing the temperature, while maintaining the high CO productselectivity. Taking the TOF (in s⁻¹) as an apparent 1^(st) order rateconstant for the CO₂ conversion (assuming steady state conditions forthe catalyst and proton source), an Arrhenius plot: In(TOF) vs 1/T (inK⁻¹) was constructed (FIG. 5B). From the slope, the activation energy,Ea, was calculated to 12 kJmol⁻¹.

In alkaline conditions (1.0 M KOH, pH 14) the cell performance wasdrastically improved. FIG. 6 shows the resulting current densities andCO product selectivity recorded at various potentials (see also FIG.12). A current density of 6 mAcm⁻² was observed at 0.014 V vs RHE, and155 mAcm⁻² was reached at −0.59 V vs RHE, with an excellent CO productselectivity, as summarized in Table 2. In a control experiment with thenon-metalated porphyrin ligand, H₂ was the only detected product (seeFIG. 13).

TABLE 2 Current densities and CO selectivity obtained with a catalyticfilm with FeTNT as catalyst (pH 14) as a function of the potentialapplied at the cathode 9. E/V vs RHE j_(av)/mAcm⁻² % CO 0.014 6.0 99.9−0.086 16.5 99.9 −0.19 37.6 99.9 −0.29 62.9 99.8 −0.39 89.7 99.5 −0.49122.4 99.2 −0.59 155.2 98.1

To investigate the endurance of the catalyst in alkaline conditions, achronopotentiometric electrolysis was performed at 27 mAcm⁻² for 24 h,the same current density value as performed in pH-neutral conditions. Asshown in FIG. 7, a potential of ca. −0.16 V vs RHE was measured over the24 hour course, showing only a minute increase. The CO productselectivity remained extremely high: 99.7±0.2% on average.

For a direct comparison with the pH-neutral 1.0 M bicarbonate system, achronopotentiometric electrolysis was performed at 50 mAcm⁻² in 1.0 MKOH, as shown in FIG. 8. Over the course a 3 hours electrolysis, theapplied potential remained stable (−0.23 V vs RHE), a very weaklynegative potential for such high current density. Again, the COselectivity was nearly perfect, with an average value of 99.8±0.1%.

Second Embodiment

The inventors included CoPc2 into the same flow cell setup described inthe first embodiment. CoPc2 was supported on a gas diffusion electrodeas the cathode 9. Details of the setup are provided in the SupportingInformation (see FIG. 16). CoPc2 was dispersed in a colloidal ink withcarbon black and subsequently deposited on the carbon fiber paper,composing the cathode 9.

At −0.3 V vs. RHE and pH 14 (1 M KOH for the electrolyte), whichcorresponds to a low 200 mV overpotential, a high current density withj_(CO)=22.2 mA/cm² was achieved (j_(CO) is the partial current densityfor CO production). The dependence of the current density for COproduction is reported in FIG. 15a (see also FIG. 22). Upon setting theelectrolysis potential at −0.72 V vs. RHE, j_(CO) raised to 111.6 mA/cm²with 96% selectivity, while excellent stability over the course of the 3h electrolysis was obtained (FIG. 15b ). The only additional gas phaseby-product was H₂ (4% selectivity) and the catholyte solution wascarefully checked by ¹H NMR; no formate nor methanol was detected duringthe experiment. The obtained j_(CO) corresponds to a turnover frequency(TOF) of 2.7 s⁻¹ and a turnover number (TON) of 29008. A maximum currentdensity of 165 mA cm⁻² for CO generation was obtained at −0.92 V vs.RHE. Long term stability of the catalytic material was assessed uponapplying a constant current density of 75 mA cm⁻² for 10 h, which led toa cathodic potential of −0.65 V vs. RHE (h=540 mV) with 94% selectivityof CO (j_(CO)=70.5 mA cm′) (FIG. 23). All the collected data arecompiled in Table 3, along with a comparison of previously reportedcatalysts. The type of cell used (H cell vs. flow cell) is alsoindicated to ease comparison between data. The Co K-edge XANES spectraof CoPc2@carbon black were recorded before and after electrocatalysis atE=−0.72 V vs. RHE. FIG. 15c shows these spectra together with that ofthe starting CoPc2 complex. All these spectra present the typicalfeatures expected for a cobalt(II) phthalocyanine complex, i.e. a lowintensity pre-edge peak at 7111 eV (corresponding to a 1s to 3d/4ptransition) and a shoulder at 7717 eV (corresponding to a 1s to 4p_(z)transition). Interestingly, the intensity of these two transitions wereshown by Li et al¹⁷ to depend on the attachment to a surface, i.e. thepre-edge intensity increases and the shoulder decreases upon adsorptiononto nanotubes, respectively. The same trend is observed in theCoPc2@carbon black system, with decreased pre-edge and increasedshoulder intensities on going from the starting complex to the adsorbedspecies. This trend continues after catalysis, suggesting an even closerinteraction with the surface while maintaining the overall structure ofthe molecular catalyst after the experiment. This structuralconservation is further confirmed by the EXAFS spectra (FIG. 24), whichalso present the typical features of a cobalt phthalocyanine complex.³⁸In addition, comparison of the spectrum of CoPc2@carbon black recordedafter catalysis with those of reference cobalt samples (FIG. 24) clearlyshows that the changes observed on the spectrum after catalysis areinsignificant as far as the overall structure is concerned.

Remarkably, CoPc2 remains highly selective for the CO₂-to-CO conversionacross an interval of 10 units of pH, extending from acidic (pH 4) tobasic solutions (pH 14). An averaged 92% selectivity for CO₂ reductionwith partial current density of ca. 20 mA cm⁻² were routinely obtainedin the whole domain of pH values with excellent stability over time. Inclose to neutral solutions (pH 7.3), CoPc2 is a significantly bettercatalyst than the non-substituted phthalocyanine CoPc (see Table 3,entries 2 and 3) with a ca. 25% increase in current density at similaroverpotential, but it also surpasses state-of-the art tetra-cyanosubstituted phthalocyanine (CoPc-CN, Table 3, entry 4) and unsubstitutedCo phthalocyanine polymerized around carbon nanotubes (CoPpc, Table 3,entry 5), both in terms of current density and turnover frequency.Similarly to the previously reported cobalt phthalocyanines mentionedabove, CoPc2 exhibits excellent stability over time, showing a 10.5 helectrolysis experiment, with no loss of performance.

TABLE 3 Comparison of electrolysis performances between CoPc2@carbonpowder hybrid catalyst and previously reported state-of-the artimmobilized molecular Co catalysts and Ag nanomaterial. E (V vs. RHE)j_(CO) CO [overpotential (mA TOF sel. Cell Entry Catalyst (mV)]Electrolyte cm⁻²) (s⁻¹) (%) type Ref. 1 CoPc2 −0.97 0.5M KCl 16.3 6.1 92H-cell this work [836]^(a) 2 CoPc2 −0.675 0.5M 18.1 6.8 93 H-cell thiswork [539]^(a) NaHCO³ 3 CoPc −0.675 0.5M 13.1 4.1 92 H-cell this work[546]^(a) NaHCO³ 4 CoPc-CN −0.63 0.1M 14.7 4.1 98 H-cell 15 [520] KHCO³5 CoPpc −0.61 0.5M 18 1.4 ca. 90 H-cell 16 [500] NaHCO³ 6 Coqpy −0.550.5M 19.9 12 99 H-cell 10 [440] NaHCO³ 7 CoPc2 −0.31 1M KOH 22.2 0.54 93Flow-cell this work [200]^(b) 8 CoPc2 −0.65 1M KOH 70.5 1.67 94Flow-cell this work [540]^(b) 9 CoPc2 −0.72 1M KOH 111.6 2.7 96Flow-cell this work [610]^(b) 10 CoPc2 −0.92 1M KOH 165 3.9 94 Flow-cellthis work [810]^(b) 11 CoPc-CN −0.66 1M KOH 31 / 94 Flow-cell 11 [550]12 Ag^(c) −0.81 1M KOH 156.5 / 92 Flow-cell 13 [700] Entry 1: Γ = 14.4nmol cm⁻² (pH 4, 2 h electrolysis), entry 2: Γ = 14.4 nmol cm⁻² (pH 7.3,1 h electrolysis), entry 3: Γ = 23.3 nmol cm⁻² (pH 7.3, 1 helectrolysis), entries 7-10: Γ = 0.216 μmol cm⁻² (pH 14, for 0.5, 10, 3and 0.3 h electrolysis respectively). Typical uncertainty on j_(CO) andTOF values is ± 5%. ^(a)corrected from ohmic drop (uncompensatedsolution resistance of ca. 3 W, electrode surface 0.5 cm²)^(b)uncorrected from ohmic drop ^(c)carbonate-derived Ag nano-catalyst(500 nm thickness), see reference 13 for details.

A turnover frequency up to 6.8 s⁻¹ was reached and, generally, a verysmall loading of the catalysts was necessary to obtain high j_(CO) (seefor example Table 3, entries 1-2). Long term electrolysis (10 h) inbasic conditions (pH 14) at −0.65 V vs. RHE led to an average j_(CO)close 70.5 mAcm⁻² and also illustrates the remarkable stability of thecobalt catalyst. The ability to implement CoPc2 in various pH conditionsis also a key feature that may allow for combining the Co catalyst tovarious types of anodic materials in order to decrease the overall cellpotential. In particular, the excellent performance obtained at pH 14should permit to pair the CoPc2 loaded cathode 9 with the most efficientoxygen evolving metal oxide anode 2 materials. At this pH, CoPc2 matchesthe state-of-the art Ag based catalyst sputtered onto a PTFE membrane,both in terms of selectivity and current density (Table 3, compareentries 10 and 12).

Upon introducing a positively charged trimethylammonium group on theparent cobalt phthalocyanine, a highly efficient and versatile catalystfor the CO₂-to-CO electrochemical conversion in water has been obtained.Furthermore, it operates with high selectivity (92 to 96%) in a broadrange of pHs, extending from acidic (pH 4) to basic conditions (pH 14).In acid and neutral conditions, current densities close to 20 mA cm⁻²were routinely obtained. In a 1 M KOH electrolyte solution, same currentdensities could be obtained at a very low overpotential of 200 mV, oncethe hybrid catalyst mixed with carbon support was included in a gas flowcell. At −0.92 V vs. RHE, a partial current for CO production of 165mAcm⁻² was found with 94% catalytic selectivity. These performances showthat hybrid catalytic materials including only carbon and an earthabundant metal based molecular complex can rival noble metalnanomaterials such as Ag and Au. This study highlights that rationaltuning of the structure of simple metal complexes may allow for highperformance, and it is likely that further improvement is yet to come.Finally, this work opens new perspectives for the development oflow-cost catalytic materials to be included in CO₂ electrolysers.

Supplementary Information Methods

All electrocatalytic reactions were carried out under an atmosphere ofargon or CO₂. Chemicals, including CoPc1 and supporting electrolyteswere obtained from commercial suppliers. Supplementary Informationsection describes the synthesis and characterization of CoPc2 and FeTNT,as well as typical protocols for electrocatalytic CO₂ reduction.

Typical protocol for electrocatalytic CO₂ reduction. After preparationof a catalytic ink containing either FeTNT, CoPc1 or CoPc2, anddeposition of the material onto porous carbon paper, controlledpotential electrolysis were performed either in a closed electrochemicalcell or in a flow cell electrolyser using a PARSTAT 4000 potentiostat(Princeton Applied Research). Gas chromatography analyses of gas evolvedin the headspace during the electrolysis were performed with an AgilentTechnologies 7820A GC system equipped with a thermal conductivitydetector. Conditions allowed detection of both H₂, O₂, N₂, CO, and CO₂.Calibration curves for H₂ and CO were determined separately by injectingknown quantities of pure gas

General Considerations Chemicals and Characterization Methods

Chemicals and materials were purchased from Sigma-Aldrich, Fluka, TCIAmerica, ABCR or Alfa Aesar, and used as received. All aqueous solutionswere prepared with Millipore water (18.2 MO cm). The MWCNTs werepurchased from Sigma-Aldrich (O.D.×L 6-9 nm×5 μm, >95%). The cobalt (II)phthalocyanine (CoPc1) ((3-form, dye content 97%) was purchased fromSigma-Aldrich. Toray Carbon Paper (CAS Number: 7782-42-5), TGP-H-60,19×19 cm was purchased from Alfa Aesar and used for preparation of thecathode 9 s for the electrochemical cell. The cathode 9 s (gas diffusionelectrodes) used in the flow cell were prepared using Freudenberg C24H5carbon paper (21×29.7 cm, product code FSGDL). VULCAN® XC72R SpecialityCarbon Black was purchased from Cabot Corporation.

4-tert-butylphthalonitrile was obtained from TCI America.3-nitrophthalonitrile was purchased from ABCR. All solvents were ofsynthetic grade. Infrared spectra (IR) were recorded on a Bio-Rad FTS175C FTIR spectrophotometer. UV-visible absorption spectra were obtainedusing a Shimadzu 2001 UV spectrophotometer. High resolution mass spectrawere measured on an Agilent 6530 Accurate-Mass Q-TOF LC/MS spectrometerequipped with electrospray ionization (ESI) source. NMR spectra wererecorded in deuterated chloroform (CDCl₃) and THF-d₈ on a Varian 500 MHzspectrometer. Melting points were recorded on a Stuart SMP apparatus.

Synthesis of FeTNT

A solution of 5,10,15,20-tetrakis(4-trimethylammonio-phenyl)porphyrintetrachloride (250 mg, 0.253 mmol, 1 eq) in water (500 mL) is preparedand put under argon flux, Mohr's salt is added to the solution (810 mg,3.75 mmol, 15 eq) and the solution is heated to 85° C. for 4 h. Ammoniumhexafluorophosphate (2.5 g, 15 mmol, 60 eq) is added and the obtainedprecipitate is separated by centrifugation (10000 rpm, 30 min). The redsolid obtained is rinsed one time in pure water and separated again bycentrifugation (10000 rpm 30 min), then rinsed one time in 1 to 1mixture of chloroform and acetone and separated by centrifugation (10000rpm 30 min). Residual solvent is evaporated under Argon flux. Yield: 92%(343 mg)

UV-Vis (DMF): λ_(max) nm (log ε) 407 (4), 568 (4.98).

Cobalt catalyst CoPc2 was prepared as illustrated here:

Synthesis of Phthalonitrile and Phthalocyanines Synthesis of3-(dimethylamino)phthalonitrile 1

3-Nitrophthalonitrile (2 g, 11.5 mmol) and dimethylamine hydrochloride(2.8 g, 34.6 mmol) were dissolved in anhydrous DMF (30 mL) under argonatmosphere then finely powdered dry potassium carbonate (30 g, 217.5mmol) was added portion-wise over 15 min. The reaction mixture wasstirred under argon at 65° C. for 24 h then poured into water (250 mL).The resulting solid was collected by filtration and washed with water.After drying in vacuum, the crude product was recrystallized fromethanol. Yield: 80% (1.57 g). m.p. 105° C. ¹H NMR (500 MHz, CDCl₃): δ,ppm 3.18 (s, 6H), 7.12 (d, 1H), 7.15 (d, 1H), 7.46 (t, 1H). ¹³C NMR (125MHz, CDCl₃): δ, ppm 172.4, 155.4, 133.1, 123.0, 120.5, 118.0, 116.6,116.3, 110.0, 100.6, 42.7. FT-IR (v, cm⁻¹): 2992, 2937, 2874, 2203,1584, 1486, 1425, 1357, 1244, 1192, 1122, 1008, 792, 722.

Synthesis of Phthalocyanine 3

Granules of lithium were added to anhydrous n-pentanol (10 mL). Thismixture was heated to 60° C. under argon flux until total consumption ofthe granules. 3-(dimethylamino)phthalonitrile 1 (0.25 g, 1.46 mmol) and4-tert-butylphthalonitrile 2 (3.2 g, 17.52 mmol) were then added andthis reaction mixture was refluxed for 18 h, then cooled to roomtemperature and poured to an ethanol/water mixture. The resulting darkblue-green precipitate was filtered off, washed several times with waterand dried. Phthalocyanine 3 was isolated from this crude mixture ofphthalocyanines by chromatography on silica gel using a mixture ofCH₂Cl₂/EtOH (100:1) as the eluent. The tetra-tert-butylphthalocyanine 4was first eluted and the phthalocyanine 3 was the second elutedcompound. Yield: 15% (160 mg). ¹H NMR (500 MHz, THF-d₈): δ, ppm −2.04(s, 1H), −1.95 (s, 1H), 1.84 (m, 10H), 1.88-1.94 (m, 17H), 3.70 (s, 3H),7.45 (d, 1H), 7.79 (m, 1H), 8.10-8.40 (m, 4H), 8.61 (d, 1H), 8.85-8.91(m, 1H), 8.98-9.24 (m, 4H), ¹³C NMR (125 MHz, THF-d₈): δ, ppm 152.97,152.94, 152.88, 152.84, 152.61, 152.58, 150.16, 150.12, 133.67, 129.92,127.43, 127.30, 127.22, 126.75, 125.50, 122.12, 122.07, 121.93, 121.90,121.77, 118.69, 118.65, 118.51, 118.48, 118.41, 117.87, 117.83, 114.74,78.54, 78.28, 78.02, 44.36, 39.98, 37.15, 37.06, 35.67, 35.60, 35.50,31.89, 31.43, 31.32, 29.65, 29.32, 22.58, 13.43. ESI-HRMS: m/z 726.4030[M]⁺ calculated for C₄₆H₄₇N₈: 725.95. UV-vis (DMF): λ_(max) nm (log ε)346 (3.90), 689 (4.87), 718 (5.38). FT-IR (v, cm⁻¹): 3288, 2955, 2863,2776, 1618, 1501, 1316, 1186, 1014, 828, 742.

Synthesis of Phthalocyanine 5

A mixture of phthalocyanine 3 (50 mg, 0.06 mmol), CoCl₂ (18 mg, 0.12mmol), and DBU (1 mL) in dried n-pentanol (5 mL) was heated to refluxfor 18 h under argon. After cooling to room temperature, the reactionmixture was poured into an ethanol/water mixture. The resultingprecipitate was filtered off and washed several times with water.Phthalocyanine 5 was purified by chromatography on silica gel using amixture of CH₂Cl₂/EtOH (50:1) as the eluent. Yield: 75% (35 mg).ESI-HRMS: m/z 782.3234 [M]⁺ calculated for C₄₆H₄₅CoN₉: 782.86. UV-vis(DMF): λ_(max) nm (log ε) 332 (3.70), 693 (4.74); FT-IR (v, cm⁻¹): 2955,2856, 1606, 1457, 1316, 1254, 1094, 804, 737.

Synthesis of phthalocyanine CoPc2

Phthalocyanine 5 (35 mg, 0.044 mmol) was dissolved in DMF (5 mL), andmethyl iodide (0.2 g, 1.5 mmol) was added. The mixture was stirred atroom temperature for 16 h then poured into diethyl ether (25 mL). Theresulting blue precipitate was filtered off, washed with ether anddried. Yield: 32 mg (88%). ESI-HRMS: m/z 797.3668 [M]⁺ calculated forC₄₇H₄₈CoN₈: 797.90. Anal. calcd for C₄₇H₅₈ColN₉O₅ (CoPc2.5H₂O): C,55.28; H, 5.98; N, 11.88. Found: C, 55.62; H, 5.76; N, 12.42. UV-vis(DMF): λ_(max) nm (log ε) 326 (4.41), 664 (4.65). FT-IR (v, cm⁻¹) 3047,2949, 2851, 1655, 1606, 1476, 1328, 1254, 1088, 933, 829, 749.

Preparation of the Hybrid Materials

For CV experiments and electrolysis in the closed electrolysis cell, 3mg of MWCNTs were dispersed in 2 mL ethylene glycol (EG)/ethanol (EtOH)1:1 (v/v) mixture followed by 30 min of sonication. 1 mg of the cobaltcatalyst (CoPc1, CoPc2) was dissolved in 1 mL EG/EtOH mixture. Variousvolumes of this solution were added to the MWCNTs suspension in a totalvolume of 3 mL, so as to get mass ratio (1:6, 1:15 and 1:30) of thecatalyst. The suspension was further sonicated for 30 min. Finally,Nafion® was added (2.9%, 30 μL) and the complete mixture was sonicatedfor 30 min to obtain the final catalytic ink.

For the flow cell set-up, 3 mg of carbon black were dispersed in 3 mLEtOH followed by 30 min of sonication. 0.2 mg of CoPc2 was dissolved in1 mL EtOH so as to get a mass ratio (1:15) of the catalyst. Thesuspension was further sonicated for 30 min. Finally, Nafion® was added(2.9%, 30 μL) and the complete mixture was sonicated for 30 min toobtain the final catalytic ink. The ink was drop casted on carbon papermasked with a Teflon frame to obtain an electrode area of 1×1 cm².

Electrochemical Studies

Controlled potential electrolyses were performed using a PARSTAT 4000potentiostat (Princeton Applied Research).

Preparative Scale Electrolysis

In the closed cell, experiments were carried out in a cell using a Toraycarbon paper as working electrode, and a SCE reference electrode closelypositioned one from the other. The Pt grid counter electrode wasseparated from the cathodic compartment with a glass frit. The catalyticink was dropped on one face of the Toray carbon paper cathode 9 (100 μLfor a 0.5 cm² electrode), and allowed to dry under ambient conditionsprior to use. The full cell setup was identical to the one usedpreviously.¹⁸

The flow cell electrolyzer 1 (Micro Flow Cell® purchased by Electrocell)is composed by a sandwich of flow frames, electrodes, gaskets and amembrane, which, when assembled as illustrated in FIG. 14, constitute athree-compartment flow cell. One compartment delivers the CO₂ (at 16.7sccm) from the back side and through the gas diffusion electrode (GDE,1×1 cm², fixated in a Pt frame), while another directs the catholytesolution (1 M KOH, flow rate of 16 sccm) in between the GDE and theanion exchange membrane 10 (AEM, Sustainion™ X37-50). On the other sideof the latter, the anolyte (1 M KOH, flow rate of 16 sccm) is directedbetween the AEM and the Pt/Ti alloy anode 2. The flow frames are made ofPTFE, and the gaskets of peroxide cured EDPM. Catholyte and anolyte wererecycled using peristaltic pumps. All tubings were made of PTFE andconnected to the cell with PEEK ferrules and fittings. The whole setupis schematically shown below (FIG. 16).

Gas Detection

Gas chromatography analyses of gas sampled from the headspace during theelectrolysis were performed with an Agilent Technologies 7820A GC systemequipped with a thermal conductivity detector. CO and H₂ production wasquantitatively detected using a CP-CarboPlot P7 capillary column (27.46m in length and 25 μm internal diameter). Temperature was held at 150°C. for the detector and 34° C. for the oven. The carrier gas was argonflowing at 9.5 mL/min at constant pressure of 0.4 bars. Injection wasperformed via a 250-μL gas-tight (Hamilton) syringe previously degassedwith CO₂. Conditions allowed detection of both H₂, O₂, N₂, CO, and CO₂.Calibration curves for H₂ and CO were determined separately by injectingknown quantities of pure gas.

XPS Analysis

An X-Ray Photoelectron Spectrometer THERMO-VG ESCALAB 250 (RX source KAl (1486.6 eV)) was used.

XAS Data Collection and Analysis

X-ray absorption spectra (XAS) were collected at the LUCIA beamline ofSOLEIL with a ring energy of 2.75 GeV and a current of 490 mA. Theenergy was monochromatized by means of a Si(111) double crystalmonochromator. Data were collected in a primary vacuum chamber asfluorescence spectra with an outgoing angle of 5° using a Bruker silicondrift detector. The data were normalized to the intensity of theincoming incident energy and processed with the Athena software from theIFEFFIT package. For the EXAFS analysis, an E₀ value of 7722.0 eV wasused for the cobalt K-edge jump energy.

SEM Analysis

Scanning electron microscopy using a field emission gun (SEM-FEG) wasperformed using a Zeiss Supra 40.

Third Embodiment

The inventors tested the cobalt quarter pyridine (Co(qpy)) in the samecell as the first embodiment. Two types of experiments have beenperformed on the Co(qpy). The faradaic efficiency for CO has beenmeasured chronopotentiometry at several current densities and thestability of the catalyst has been determined at 50 mA cm⁻².

During chronopotentiometry experiment the Co(qpy) showed faradaicefficiencies for CO higher than 90% at current densities under 100 mAcm⁻². Then, at higher current densities, the faradaic efficiency for COdecreases drastically.

TABLE 4 Chronopotentiometry experiment with Co(qpy) Cell potential (V)j/mA · cm⁻² % CO 1.71  25 96.6 1.80  50 98.7 1.88  75 98.8 2.24 100 91.62.60 125 86.1 2.71 150 64.6 2.73 175 34.7 2.74 200 16.6

In FIG. 26, Co(qpy) can maintain at 50 mAcm⁻² a faradaic efficiencyhigher than 90% for CO for 12 hours. The decrease is steady until 24hours of experiments. Then, the faradaic efficiency drops under 40% forCO.

Inventors also tested influence of the electrolyte (cation) used in theflow cell electrolyzer. Results are shown in the Table 5 below.

TABLE 5 Influence of the electrolyte CO selectivity E (V vs EnergyEfficiency Catalyst Cation (%) RHE) (%) CoPc Na 88.2 −1.10 29 CoPc Cs92.3 −1.00 33 Co(qpy) Na 97.8 −1.01 33 Co(qpy) Cs 98.7 −0.90 35 FeTNT Na98.3 −1.14 33 FeTNT K 98.6 −0.95 36 FeTNT Cs 98.9 −0.75 41

REFERENCES

-   1. Azcarate, I., Costentin, C., & al. Through-space charge    interaction substituent effects in molecular catalysis leading to    the design of the most efficient catalyst of 002-to-CO    electrochemical conversion. J. Am. Chem. Soc. 138, 16639-16644    (2016).-   2. Francke, R., Schille, B., & al. Homogeneously catalyzed    electroreduction of carbon dioxide-Methods, mechanisms, and    catalysts. Chem. Rev. 116, 4631-4701 (2018).-   3. Costentin, C., Robert, M., Savéant, J.-M. Catalysis of the    electrochemical reduction of carbon dioxide. Chem. Soc. Rev. 42,    2423-2436 (2013).-   4. Qiao, J., Liu, Y., & al. A review of catalysts for the    electroreduction of carbon dioxide to produce low-carbon fuels.    Chem. Soc. Rev. 43, 631-675 (2014).-   5. Elgrishi, N., Chambers, M. B., & al. Molecular polypyridine-based    metal complexes as catalysts for the reduction of CO₂. Chem. Soc.    Rev. 46, 761-796 (2017).-   6. Grice, K. A. Carbon dioxide reduction with homogenous early    transition metal complexes: Opportunities and challenges for    developing CO₂ catalysis. Coord. Chem. Rev. 336, 78-95 (2017).-   7. Grills, D. C., Ertem, M. Z., & al. Mechanistic aspects of CO₂    reduction catalysis with manganese-based molecular catalysts. Coord.    Chem. Rev. 374, 173-217 (2018).-   8. Loewen, N. D., Neelakantan, T. V., & al. Renewable formate from    C—H bond formation with CO₂: using iron carbonyl clusters as    electrocatalysts. Acc. Chem. Res. 50, 2362-2370 (2017).-   9. Takeda, H., Cometto, C., & al. Electrons, photons, protons and    earth abundant metal complexes for molecular catalysis of CO₂    reduction. ACS Catal. 7, 70-88 (2017).-   10. Wang, M., Chen, L., Lau, T-C., Robert, M. Hybrid Co    quaterpyridine complex/carbon nanotube catalytic material for CO₂    reduction in water. Angew. Chem. Int. Ed. 57, 7769-7773 (2018).-   11. Xu, L., Wu, Y., & al. High-Performance Electrochemical CO₂    Reduction Cells Based on Non-noble Metal Catalysts. ACS Energy Lett.    3, 2527-2532 (2018).-   12. Kutz, R. B., Chen, Q., et al., I. R. Sustainion    imidazolium-functionalized polymers for carbon dioxide electrolysis.    Energy Technol. 5, 929-936 (2017).-   13. Dinh, C-T., Garcia de Arguer, F. P., & al. H. High Rate,    Selective, and Stable Electroreduction of CO₂ to CO in Basic and    Neutral Media. ACS Energy Lett. 3, 2835-2840 (2018).-   14. Verma, S., Hamasaki, Y., & al. Insights into the low    overpotential electroreduction of CO₂ to CO on a supported gold    catalyst in an alkaline flow electrolyzer. ACS Energy Lett. 3,    193-198 (2018).-   15. Zhang, X., Wu, Z., & al., Highly selective and active CO₂    reduction electrocatalysts based on cobalt phthalocyanine/carbon    nanotube hybrid structures. Nat. Commun. 8, 14675 (2017).-   16. Han, N., Wang, Y., Ma, L. et al., Supported cobalt    phthalocyanine for high-performance electrocatalytic CO₂ reduction.    Chem 3, 652-664 (2017).-   17. Li, N., Lu, W., Pei K., Chen, W. Interfacial peroxidase-like    catalytic activity of surface-immobilized cobalt phthalocyanine on    multiwall carbon nanotubes RSC Advances, 5, 9374-9380 (2015).-   18. Wang M., Chen L. G., Lau T.-C., and Robert M. A Hybrid Co    Quaterpyridine Complex/Carbon Nanotube Catalytic Material for CO₂    Reduction in Water, Angew. Chem. 130, 7895-7899, (2018).

1. A flow cell electrolyzer to electrochemically reduce CO₂ in aCO₂-containing reagent gas, passing onto or through a gas diffusionelectrode (GDE), into gaseous CO, the flow cell electrolyzer comprising:an anodic compartment comprising: an anode with a current collector, ananodic electrolyte solution, at a controlled flow rate Q_(a),comprising: a first solvent, and an anodic electrolyte, the firstsolvent being water, an anodic electrolyte solution inlet and an anodicelectrolyte solution outlet connected to the anodic compartment, adaptedto circulate the anodic electrolyte solution; a cathodic compartmentcomprising: a cathodic electrolyte solution, at a controlled flow rateQ_(c), comprising: a second solvent, and a cathodic electrolyte, thesecond solvent being H₂O, a cathodic electrolyte solution inlet and acathodic electrolyte solution outlet connected to the cathodiccompartment, adapted to circulate the cathodic electrolyte solution, aswell as the CO₂-containing reagent gas and CO product gas, a gasdiffusion electrode comprising, on an electrochemically inert gasdiffusion porous current collector having a surface S, a molecularcatalyst to electrochemically reduce the CO₂ in the CO₂-containingreagent gas into said CO product gas in the cathodic electrolytesolution, with by-production of gaseous H₂; the molecular catalyst beingselected from the group consisting of: metal tetra phenyl porphyrinsubstituted with at least one ⁺N(C₁-C₄ alkyl)₃ group and having othergroups independently selected from the groups consisting of H, OH, F,C(CH₃)₃, wherein the metal is chosen from among: iron and cobalt, metalphthalocyanine, with the metal chosen among: iron and cobalt, metalphthalocyanine, substituted with at least one group among: ⁺N(C₁-C₄alkyl)₃, F, C(CH₃)₃ wherein the metal is chosen among: iron and cobalt,and cobalt quarter pyridine; an anion exchange membrane, impermeable atleast to CO₂, CO and H₂, between the anodic compartment and the cathodiccompartment; a channel for passing flow of the CO₂-containing reagentgas, at a controlled flow rate Q_(g), through the porous currentcollector surface S of the gas diffusion electrode of the cathodiccompartment on which there is the molecular catalyst, while the cathodicelectrolyte solution circulates in between the gas diffusion electrodeand the anion exchange membrane; pumping means adapted to: circulate bypumping the anodic electrolyte solution in the anodic compartment andthe cathodic electrolyte solution in the cathodic compartment betweenthe respective inlets and outlets thereof, control flow by pumping theCO₂-containing reagent gas in the channel, passing through the porouscurrent collector surface of the gas diffusion electrode, the pumpingmeans being configured to control all flow rates of the cathodic andanodic electrolyte solutions as well as the CO₂-containing reagent gaspassing through the porous current collector surface S of the gasdiffusion electrode; a power supply providing energy necessary totrigger the electrochemical reactions involving the CO₂ reagent gas. 2.The flow cell electrolyzer according to claim 1, wherein the anodicand/or cathodic electrolyte solution has a neutral or basic pH.
 3. Theflow cell electrolyzer according to claim 1, wherein the anodicelectrolyte solution and/or the cathodic electrolyte solution has a pHfrom 9 to
 14. 4. (canceled)
 5. The flow cell electrolyzer according toclaim 1, wherein the CO₂-containing reagent gas flow is at atmosphericpressure.
 6. The flow cell electrolyzer according to claim 1, whereinthe anodic and the cathodic electrolyte solutions are at ambienttemperature.
 7. The flow cell electrolyzer according to claim 1, whereinthe molecular catalyst is a tetra-phenyl iron porphyrin with theformula:

wherein: at least 1 and at most 8 groups among R₁ to R₁₀ and R_(1′) toR_(10′) being independently ⁺N(C₁-C₄ alkyl)₃ group, the others of groupsR₁ to R₁₀ and R_(1′) to R_(10′) are independently selected from thegroups consisting of H, OH, F, C(CH₃)₃.
 8. The flow cell electrolyzeraccording to claim 7, wherein: the others of groups among R₁ to R₁₀ andR_(1′) to R_(10′) are H, or the others of groups among R₁ to R₁₀ andR_(1′) to R_(10′) are independently selected from the groups H and F. 9.The flow cell electrolyzer according to claim 7, comprising ⁺N(C₁-C₄alkyl)₃ groups in the para or ortho position.
 10. The flow cellelectrolyzer according to claim 1, wherein the molecular catalystpresents the formula:

wherein R₁ to R₁₆ are independently selected from the groups consistingof H, F, C(CH₃)₃ and ⁺N(C₁-C₄ alkyl)₃.
 11. The flow cell electrolyzeraccording to claim 10, wherein R₁ to R₁₆ are H.
 12. The flow cellelectrolyzer according to any claim 10, at least 1 and at most 8 groupsamong R₁ to R₁₆ being independently ⁺N(C₁-C₄ alkyl)₃.
 13. The flow cellelectrolyzer according to claim 12, wherein at least one or several ofthe specific groups R₁, R₄, R₅, R₈, R₉, R₁₂, R₁₃ and R₁₆ groups,comprises a ⁺N(C₁-C₄ alkyl)₃ substituent.
 14. The flow cell electrolyzeraccording to claim 1, wherein the cathodic electrolyte solutioncomprises a phosphate buffer or potassium hydroxide, and the anodicelectrolyte solution comprises a phosphate buffer or potassiumhydroxide.
 15. The flow cell electrolyzer according to claim 1, whereinthe cathodic electrolyte solution comprises sodium hydroxide or cesiumhydroxide and/or the anodic electrolyte solution comprises sodiumhydroxide or cesium hydroxide.
 16. The flow cell electrolyzer accordingto claim 1, wherein the molecular catalyst is tetra-phenyl ironporphyrin, wherein: the pH of the anodic electrolyte solution and/or thecathodic electrolyte solution is between from 11.5 to 14, theCO₂-containing reagent gas flow passes at atmospheric pressure throughthe porous current collector surface S of the gas diffusion electrode,and the anodic and the cathodic electrolyte solutions are at ambienttemperature.
 17. The flow cell electrolyzer according to claim 10,wherein: the pH of the anodic electrolyte solution and/or the cathodicelectrolyte solution is between from 11.5 to 14, the CO₂-containingreagent gas flow passes at atmospheric pressure through the porouscurrent collector surface S of the gas diffusion electrode, and theanodic and the cathodic electrolyte solutions are at ambienttemperature.
 18. The flow cell electrolyzer according to claim 10,wherein R₁ to R₁₆ comprise at least one ⁺N(C₁-C₄ alkyl)₃ group, wherein:the pH of the anodic electrolyte solution and/or the cathodicelectrolyte solution is between from 11.5 to 14, the CO₂-containingreagent gas flow passes at atmospheric pressure through the porouscurrent collector surface S of the gas diffusion electrode (GDE), andthe anodic and the cathodic electrolyte solutions are at ambienttemperature.
 19. The flow cell electrolyzer according to claim 1,wherein the molecular catalyst is cobalt quarter pyridine, wherein: thepH of the anodic electrolyte solution and/or the cathodic electrolytesolution is between from 11.5 to 14, the CO₂-containing reagent gas flowpasses at atmospheric pressure through the porous current collectorsurface S of the gas diffusion electrode, and the anodic and thecathodic electrolyte solutions are at ambient temperature.
 20. The flowcell electrolyzer according to claim 1, wherein the gas diffusionelectrode in the cathodic compartment comprises, on the currentcollector, an electrode film which contains polymers with the molecularcatalysts and wherein the electrode film is deposited or grafted on thecurrent collector.
 21. The flow cell electrolyzer according to claim 1,wherein pumping means are configured to recirculate the anodicelectrolyte solution and the cathodic electrolyte solution. 22-31.(canceled)